1. Field of the Invention
The present invention relates to a pressurization device for the rotor of a motor, which is applied to a micro motor of the type, in which a magnet is mounted on the rotor shaft and having two layers of stator coils on its outer circumference.
2. Prior Art
One example of a conventional micro motor for use of various applications is explained in accordance with FIG. 8, in which the upper half portion thereof is shown in section to show the inner structure. Reference numeral 1 designates a stator yoke, outside which a bearing 2 is mounted and inside which a bearing 3 is mounted, which support rotatably a rotor shaft 4. On the rotor shaft 4, a sleeve 5 is mounted, around the outer circumference of which a cylindrical magnet 6 is attached. Thereby, the sleeve 5 and the magnet 6 are unified.
The length of the sleeve 5 is shorter than the spacing between the bearings 2 and 3, one end thereof is adjacent to a washer 7 through which the rotor shaft 4 is penetrated and the other end is adjacent to a plate spring 8. The washer 7 is in contact with the bearing 2 and the plate spring 8 is in contact with the bearing 3. Outside the outer circumference of the magnet 6 the inner side of the stator yoke 1 located with a little gap therebetween, and inside the yoke 1 stator coils 9 and 10 are juxtaposed.
In such a conventional micro motor thus formed, when the stator yoke 1 is energized by flowing the alternating current into the coils 9 and 10, the rotor shaft 4 rotates due to the electromagnetic force functioned on the magnet 6 in accordance with a polarizing action thereof. Letting the motor be a stepping motor, if the coils 9 and 10 are flown alternately with a pulse current, the magnet 6 is functioned alternately with the electromagnetic force. When the coils 9 and 10 are flown alternately with the pulse current, the energization of the stator yoke 1 changes on the rotor shaft 4 in the axial direction, since the rotor shaft 4 receives a to and fro force alternately in the axial direction, which causes the rotor shaft 4 to be in a unstable location and to vibrate, as a result a noise is generated.
Then, in order to avoid this, by pressing the sleeve 5 to the bearing 2 through the washer 7 due to the function of the plate spring 8 put on the end portion of the rotor shaft 4, the location of the rotor shaft 4 is fixed in the axial direction. According to the structure shown in FIG. 8, the sleeve 5 is not adapted to be in contact with the bearing 2 directly because the washer 7 on the rotor shaft 4 is interposed between the sleeve 5 and the bearing 2. Accordingly, there is no trouble on the function of the motor.
However, at the time of assembling the motor, if the plate spring 8 put on the rotor shaft 4 is not fixed, it is apt to drop when assembling the rotor shaft in the stator yoke 1. To avoid this trouble, the plate spring 8 to have been adhered by grease to the rotor shaft 4 while assembling, which is not suitable for an automatic assembling process and has to have relied on a hand working. Further, a mere replacement of the plate spring 8 with a coil spring has not improved the assembling work.
Since the spacing between the sleeve 5 of a rotor of a stepping motor and the bearing 3, that is, the dimension precision of the spacing where the plate spring 8 is accommodated depends on the precision of the length of the sleeve 5 and the precision of the spacing between the bearings 2 and 3, the tiny dimension change of the spacing influences greatly the rotor pressurization due to the plate spring 8 and a contact pressure between the sleeve 5 and the rotor shaft 4, as a result to the characteristic of the motor torque.
The relation of a variation (mm) on a load (N), as a representative example of stress variation characteristic of the plate spring, and as a representative example of the coil spring, is shown in FIG. 9 and in FIG. 10 respectively. Comparing these examples, a stress variation curve at the initial use of the plate spring is shown in 1 which becomes 2 after aging. For instance, at the variation of 0.1 mm-0.5 mm, the load is within 0.35N-1N, and in the case of a coil spring, both in 1 and 2, the load shows a little variation of 0.8N -1.2N. Accordingly, when a coil spring is employed for a motor, against the clearance of the space, in which the coil spring is accommodated, the variation of the pressurization decreases.
Therefore, a coil spring, in which the variation of the pressurization due to aging is small and the influence receiving from the space variation is small, is superior and easy to be used.
However, if such a coil spring is mounted without a coil cover or holder as well as a plate spring, the pressurized coil spring biases the rotation force of the rotor in one rotary or one reverse direction by adding a torsion force, which causes a stepping motor to be influenced badly in its angle precision indicating the position precision at every one step and in the torque characteristic.
Then, in the present invention, a coil spring is accommodated in a holder to enable the holder easily to be assembled automatically and the structure is adapted to effect for it to keep the position provisionally after being coupled with the rotor shaft, so that the motor assembly becomes easy and the structure, which does not cause the pressurization added to the rotor to influence badly against the angle precision and the torque characteristic, is provided.
The present invention described in the claim 1 is, to solve the above problem, characterized in that a cup-shaped base and cap are adapted to be coupled to accommodate therein a coil spring, which form a holder, and said holder is interposed between a bearing of the rotor shaft and a sleeve.
The invention described in the claim 2, in the invention described in the claim 1, is characterized in that said base and cap are made of plastic material, at the bottom center of one of them, a non-circular opening having the diameter a little smaller than the diameter of the rotor shaft is provided and at the opposite bottom center a circular opening having a diameter a little larger than the diameter of the rotor shaft is provided.
The invention described in the claim 3, in the inventions described in the claims 1 or 2, is characterized in that one portion of said base is provided with an elongated aperture along the rotor shaft and the cap is provided with a click to allow to slide in that aperture within a given range thereof and to engage with the end of the aperture.
The invention described in the claim 4, in either one of the inventions described in the claims 1 to 3, is characterized in that the material of said holder is made of thermoplastic resin.
The invention described in the claim 5, in the invention described in either one of the claims 1 to 4, is characterized in that said cap is adapted to be coupled with the holder with having a clearance.
Since the base and the cap of the holder is coupled with a sufficient clearance and covers a coil spring, the base and cap of the holder are freely expanded and contracted in the state where the coil spring is accommodated therein to function resiliently. Further, since the diameter of the opening for inserting the rotor shaft and holding the holder thereon is a little smaller than the one of the rotor shaft, when the holder is put on the rotor shaft, it is in the state of half-fixed and held provisionally thereon, so that it is not easily removed therefrom.